High temperature, high stress machine applications such as gas turbine engines have required the development of nickel and cobalt based superalloys. Components formed of such alloys may be cast to be equiaxed (random polycrystalline structure), or to be columnar grained (crystal grains formed parallel to a major stress axis), or to be a single crystal (no grain boundaries). Columnar grained and single crystal structures are formed by directionally solidifying molten alloy material during the casting process, and such structures can provide performance benefits for certain applications.
It is desired to repair a directionally solidified superalloy component rather than to replace it in order to reduce cost. However, repair of such materials is difficult because the repair process can destroy the directionality of the underlying crystal structure, thereby weakening the component.
U.S. Pat. No. 8,141,769 discloses a repair process for directionally solidified materials wherein a solder is applied in the repair region at a temperature which is low enough not to change the crystal structure of the underlying substrate material, and a temperature gradient is induced to produce a directionally solidified grain structure in the solder material. While this process preserves the underlying grain structure, it is limited to local repairs having a width of 1-1,000 μm. Furthermore, the need for a low melting temperature constituent in the solder limits the selection of materials that may be used for the repair.
U.S. Pat. No. 7,784,668 discloses the use of a preform shape of repair material that is melted and allowed to solidify onto a directionally solidified substrate, thereby preferentially seeding and orienting with the substrate material grains. However, the thickness of the preform shape must be limited due to the fluidity and limited surface tension of the molten additive material. Thicker repairs must be accomplished by sequentially applying multiple preform shapes in a series of repetitive, duplicative steps, or otherwise a container or mold must be provided to support the repair material in its molten state.
Superalloy airfoils of gas turbine engines are most commonly repaired by incrementally depositing layers of repair material onto the airfoil substrate surface with a welding or cladding process. The repair material is selected to match the substrate material or to have similar high temperature properties. Such cladding repairs may be accomplished with gas tungsten arc welding (GTAW) using wire as the filler material, or for lower heat applications, with microplasma arc welding (PAW) or laser beam welding (LBW) usually using powder material as the filler material. Many variations of this technology have been developed, including preweld heat treat conditioning of the substrate, elevated temperature preheating of the substrate, and post weld heat treatments such as hot isostatic pressing (HIPing). However, such welding processes fail to replicate the microstructure of the underlying substrate, and thus they are unable to produce materials properties equal to those achieved in the original component.